Microelectrode recording was carried out in the thalamic reticularis nucleus (Rt) during 51 stereotaxic operations performed in locally anesthetized dyskinetic patients. The spontaneous activity (SA) of 426 units was studied by means of computer processing techniques. Three types of unit (A, B, C) were shown to exist in Rt: with irregular low-frequency (0-10/sec) discharges (A type, 51%); bursting in short trains (10-30 msec) with unstable rhythmic pattern (2-5/sec; B type, 42%); presenting long duration (0.1-2 sec) high frequency bursts and relatively constant interburst silences (80-150 msec; C type, 7%). During short-term anesthesia A unit discharges disappeared; on the contrary the rhythmic bursts of B neurons were synchronized and presented a more stable frequency. The 3 types of cell were present in the whole Rt. However, a number of discharge characteristics (frequency, variation of rhythm) of A and B units changed significantly with the position of the cells in the Rt. No relationship was found between the frequencies of the rhythmic bursts and the parkinsonian tremor. With the use of a multiparametric statistical procedure, a relation was, however, found between the intensity of the peripheral tremor and the stability of the average frequency of the B type rhythmic bursts. The possible origins of rhythmic bursts of B and C neurons are discussed.